1. Field of the Invention
The present invention relates generally to the technical field of dentistry and, more particularly, to a dental implant installed into a patient's jaw to support a dental prosthesis.
2. Description of the Prior Art
U.S. Pat. No. 2,721,387 which issued Jul. 13, 1953, to Edward S. Ashuckian ("the Ashuckian Patent") discloses various different structures that are adapted for implantation into a socket from which a tooth has just been extracted. The various implants disclosed in the patent are shaped to fill the cavity previously occupied by the root of the extracted tooth. The Ashuckian Patent cautions that an X-ray must be taken prior to extracting the natural tooth to determine the shape and location of the root system so that an implant properly shaped for the socket is at hand when the tooth is extracted. The Ashuckian Patent states that its various structures all lend themselves to ready, rapid, and firm integration with the surrounding structure. In particular, FIG. 9 of the Ashuckian Patent discloses an implant in which, after its insertion into the socket of the extracted tooth, rotation of a threaded screw draws a nut toward the middle of the implant thereby spreading apart two legs then located within the socket. Nevertheless, the Ashuckian Patent acknowledges that a crown may not be placed on the implant until the healing process, which integrates the implant into the jaw, is well advanced or completed. The Ashuckian Patent cautions that if the implant is not held firmly in place within the jaw, it will move and work in the socket, and enlarge and irritate the surrounding structure.
U.S. Pat. No. 3,579,831 which issued May 25, 1971, to Irving J. Stevens ("the Stevens Patent") discloses an elongated dental implant which threads into a cylindrically-shaped bore formed into a jawbone. One end of the implant includes two self-tapping threaded surfaces that are divided along their length by an elongated notch or slit. The slit permits the threaded ends of the implant to flex resiliently toward and away from each other during insertion into the bore thereby enhancing the implant's self-threading action. The other end of the implant is formed to provide a fastening means to which a crown may be secured, and to concurrently provide an attachment location for a tool used in threading the implant into the bore. The implant disclosed in the Stevens Patent further includes at least one stabilizing pin that passes obliquely through the implant into the jawbone which prevents the implant's rotation. The Stevens Patent discloses that this implant, including its pin, addresses a problem of implant stability, and resists loosening of the implant due to stresses and vibrations.
U.S. Pat. No. 3,708,883 which issued Jan. 9, 1973, on an application filed by Stanley Flander ("the Flander Patent") discloses an implant which includes an elongated tubular body having a pair of extensions formed with inner surfaces which diverge away from each other. A spreader screw, having a T-shaped head which engages the diverging inner surfaces of the tubular body, extends along the body's entire length to project out the end of the body furthest from the diverging extensions. After the body's extensions and the T-shaped spreader screw have been inserted into a bore formed into the jawbone of a patient, rotating a nut threaded onto the end of the spreader screw that projects out of the bore beyond the tubular body draws the T-shaped head of the spreader screw along the length of the body thereby forcing the extensions to spread apart within the bore formed in the jawbone.
U.S. Pat. No. 4,431,416 which issued Feb. 14, 1984, on an application filed by Gerald A. Niznick ("the Niznick Patent") discloses a threaded implant having a lower end which is formed with a hollow, perforated cylindrically-shaped core. A bore is prepared in the jawbone to receive the implant by first forming a hollow cavity having a depth of about one-half the implant's ultimate penetration into the bone. The remainder of the bore is then formed into the jawbone using a special trephine drill which leaves a bone core that mates with the interior of the implant's perforated core. After the implant is screwed into the bore, bone growth or regrowth occurs through the perforations in the implant's core. While the Niznick Patent acknowledges that screw-type dental implants are advantageous because they can immediately support a structural connection, the patent's text expressly states that after the implant installation the gum tissue is to be sutured together over the implant's site for some number of weeks while bone growth or regrowth occurs. Only after this bone growth or regrowth occurs may the implant support a prosthesis. The patent also discloses that engagement between the surrounding bone and the uppermost turn of the implant's external threads blocks infiltration of foreign matter further into the bore. The patent discloses that an implant of this type requires a minimum penetration into the bone of at least 9.5 millimeters.
U.S. Pat. Nos. 5,004,421 and 5,087,199 which respectively issued on Apr. 2, 1991, and on Feb. 11, 1992, based upon an application initially filed on Jul. 27, 1990, by Sargon Lazarof ("the Lazarof Patents"), both disclose an elongated tubular body which fits into a preformed bore in the jawbone. The body includes two sets of internal threads respectively located at each end of the body. Four radial slits are formed along the length of the body at one end which receives a threaded expander. The body also includes external self-taping threads which thread into the bone. To install the implant, the end of the body holding the expander screws into the bore in the jawbone until it bottoms out against the lower surface of the bore. A hexagonal wrench is then inserted into a socket in the expander and the expander is rotated to draw it toward the middle of the body. Movement of the expander toward the middle of the body spreads the portion of the body enclosing the expander outward into the surrounding bone. The body's internal and external threads have opposite handedness so rotation of the expander within the body tends to screw the body deeper into the jawbone rather than out of the jawbone.
Many dental implants being used today, such as that disclosed in the Niznick Patent, do not immediately accept functional loading, e.g. the forces of chewing food. With such implants, as long as six months may elapse between installation of the implant into a patient's jaw and installation of a prosthesis. During this extended interval of time bone regrows around and into an initially loose implant until it to becomes firmly fixed within the jaw. Installing a prosthesis after this long healing period requires a second surgery to expose the head of the implant before attaching the prosthesis. Expanding, screw type dental implants, such as that disclosed in the Flander and Lazarof Patents, attempt to provide an implant that will immediately accept functional loading. Presently, there exist no commercially available dental implants, such as those disclosed in the Ashuckian, Flander and Lazarof Patents, which expand during placement. Furthermore, it appears that the United States Food and Drug Administration ("FDA") has, thus far, not approved any such implants for general use.
Another disadvantage of present commercially available dental implants is that placement at particular locations is inhibited or made more complicated by their excessive length. Placing a long implant in zones having minimal depth of bone, particularly in the posterior maxilla and mandible, can be difficult. In the posterior maxilla, an excessively long implant encroaches on the maxillary sinus. In the posterior mandible, an excessively long implant encroaches on the mandibular neuro-vascular bundle.
One disadvantage of an expanding implant such as those disclosed in the Ashuckian, Flander, and Lazarof Patents is that upon installation they establish a void at the end of the implant deepest within the jaw. Another disadvantage of an expanding implant such as those disclosed both in the Ashuckian, Flander and Lazarof Patents is that they provide passages or openings between the end of the implant deepest within the jaw and the end of the implant which receives a prosthesis. Such passages through the implant may provide an avenue for bacterial infection. Furthermore, it is difficult to collapse an expanding implant such as that disclosed in the Ashuckian and Lazarof Patents immediately or shortly after installation if the implant's removal should become necessary.